Fix number strings being converted to numbers

Fixes mohsen1#78. This also fixes numbers larger than
Number.MAX_SAFE_INTEGER represented as strings being rounded
when the yaml is loaded and serialized again.

Causes a slight regression with the styling of string values.
Specifically, any strings that are modified in the JSON are quoted
in the resulting yaml, even if they weren't in the original.

src/index.js

@@ -293,7 +293,7 @@ function updateMap(ast, newJson, json, yaml, offset) {
 */
 function replacePrimitive(node, value, yaml, offset) {
   return yaml.substr(0, node.start_mark.pointer + offset) +
-    String(value) +
+    JSON.stringify(value) +
     yaml.substring(node.end_mark.pointer + offset);
 }
 
@@ -417,6 +417,10 @@ function indent(str, depth) {
  *
 */
 function cleanDump(value) {
+  if (value === null || isString(value) || isNumber(value)) {
+    return value;
+  }
+
   let yaml = dump(value).replace(/\n$/, '');
 
   if (EOL !== '\n') {

test/object.js

@@ -210,7 +210,7 @@ describe('preserves comments and styling in objects when', ()=> {
       expect(yawn.yaml).to.equal(`
       # leading comment
       a:
-        x: abc
+        x: "abc"
       b:
         y: 'b'
         z: abc

test/string.js

@@ -18,7 +18,7 @@ describe('preserves comments and styling when', ()=> {
 
       expect(yawn.yaml).to.equal(`
         # leading comment
-        newValue # inline comment
+        "newValue" # inline comment
         # trailing comment`);
     });
 
@@ -36,5 +36,20 @@ describe('preserves comments and styling when', ()=> {
         null # inline comment
         # trailing comment`);
     });
+
+    it('string numbers stay as numbers', ()=> {
+      let str = `
+        # leading comment
+        foo: "1000" # inline comment
+        # trailing comment`;
+
+      let yawn = new YAWN(str);
+      yawn.json = { foo: "1001" };
+
+      expect(yawn.yaml).to.equal(`
+        # leading comment
+        foo: "1001" # inline comment
+        # trailing comment`);
+    });
   });
 });