Fixes vnd header regression by changing Headers encoding

I mistakenly advised `@Headers` to follow the encoding rules of `@Body`.
This was a a mistake as in both cases, url encoding is a bad choice, if
the only goal is to prevent accidental variable expansion. For example,
url encoding interferes a lot with content, including messing with '+'
characters, such as exist in "Accept: application/vnd.github.v3+json"

This changes `@Headers` to only address the problem, which where a '{'
literal is desired in a header value. The solution offered here is to
simply repeat "{{" when you desire a '{' literal. For example, if your
header value needs to be literally "{{variable}}", you'd encode it as
"{{{{variable}}".

The impact of this change is limited to those who have already started
using v8.15, and a fast release will occur after merge to limit that.

See #326
Fixes #345
Closes #346

core/src/main/java/feign/RequestTemplate.java

@@ -60,7 +60,6 @@ public final class RequestTemplate implements Serializable {
   private boolean decodeSlash = true;
 
   public RequestTemplate() {
-
   }
 
   /* Copy constructor. Use this when making templates. */
@@ -128,9 +127,19 @@ public static String expand(String template, Map<String, ?> variables) {
     for (char c : template.toCharArray()) {
       switch (c) {
         case '{':
+          if (inVar) {
+            // '{{' is an escape: write the brace and don't interpret as a variable
+            builder.append("{");
+            inVar = false;
+            break;
+          }
           inVar = true;
           break;
         case '}':
+          if (!inVar) { // then write the brace literally
+            builder.append('}');
+            break;
+          }
           inVar = false;
           String key = var.toString();
           Object value = variables.get(var.toString());
@@ -209,13 +218,11 @@ public RequestTemplate resolve(Map<String, ?> unencoded) {
     }
     url = new StringBuilder(resolvedUrl);
 
-    Map<String, Collection<String>>
-        resolvedHeaders =
-        new LinkedHashMap<String, Collection<String>>();
+    Map<String, Collection<String>> resolvedHeaders = new LinkedHashMap<String, Collection<String>>();
     for (String field : headers.keySet()) {
       Collection<String> resolvedValues = new ArrayList<String>();
       for (String value : valuesOrEmpty(headers, field)) {
-        String resolved = urlDecode(expand(value, encoded));
+        String resolved = expand(value, unencoded);
         resolvedValues.add(resolved);
       }
       resolvedHeaders.put(field, resolvedValues);
@@ -285,37 +292,37 @@ public String url() {
   }
 
   /**
-   * Replaces queries with the specified {@code configKey} with url decoded {@code values} supplied.
+   * Replaces queries with the specified {@code name} with url decoded {@code values} supplied.
    * <br> When the {@code value} is {@code null}, all queries with the {@code configKey} are
    * removed. <br> <br><br><b>relationship to JAXRS 2.0</b><br> <br> Like {@code WebTarget.query},
    * except the values can be templatized. <br> ex. <br>
    * <pre>
    * template.query(&quot;Signature&quot;, &quot;{signature}&quot;);
    * </pre>
    *
-   * @param configKey the configKey of the query
+   * @param name the name of the query
    * @param values    can be a single null to imply removing all values. Else no values are expected
    *                  to be null.
    * @see #queries()
    */
-  public RequestTemplate query(String configKey, String... values) {
-    queries.remove(checkNotNull(configKey, "configKey"));
+  public RequestTemplate query(String name, String... values) {
+    queries.remove(checkNotNull(name, "name"));
     if (values != null && values.length > 0 && values[0] != null) {
       ArrayList<String> encoded = new ArrayList<String>();
       for (String value : values) {
         encoded.add(encodeIfNotVariable(value));
       }
-      this.queries.put(encodeIfNotVariable(configKey), encoded);
+      this.queries.put(encodeIfNotVariable(name), encoded);
     }
     return this;
   }
 
   /* @see #query(String, String...) */
-  public RequestTemplate query(String configKey, Iterable<String> values) {
+  public RequestTemplate query(String name, Iterable<String> values) {
     if (values != null) {
-      return query(configKey, toArray(values, String.class));
+      return query(name, toArray(values, String.class));
     }
-    return query(configKey, (String[]) null);
+    return query(name, (String[]) null);
   }
 
   private String encodeIfNotVariable(String in) {

core/src/test/java/feign/RequestTemplateTest.java

@@ -154,14 +154,26 @@ public void resolveTemplateWithHeaderSubstitutionsNotAtStart() {
   }
 
   @Test
-  public void resolveTemplateWithHeaderWithURLEncodedElements() {
+  public void resolveTemplateWithHeaderWithEscapedCurlyBrace() {
     RequestTemplate template = new RequestTemplate().method("GET")
-        .header("Encoded", "%7Bvar%7D");
+        .header("Encoded", "{{{{dont_expand_me}}");
 
-    template.resolve(mapOf("var", "1234"));
+    template.resolve(mapOf("dont_expand_me", "1234"));
 
     assertThat(template)
-        .hasHeaders(entry("Encoded", asList("{var}")));
+        .hasHeaders(entry("Encoded", asList("{{dont_expand_me}}")));
+  }
+
+  /** This ensures we don't mess up vnd types */
+  @Test
+  public void resolveTemplateWithHeaderIncludingSpecialCharacters() {
+    RequestTemplate template = new RequestTemplate().method("GET")
+        .header("Accept", "application/vnd.github.v3+{type}");
+
+    template.resolve(mapOf("type", "json"));
+
+    assertThat(template)
+        .hasHeaders(entry("Accept", asList("application/vnd.github.v3+json")));
   }
 
   @Test