Fix regression where only first backslash/apostrophe was escaped

d152b2d broke pretty much all usage
of Erubi on websites because it only escapes the first backslash
or apostrophe in the text when escaping.  This removes the use
of String#[]= to handle escaping, moving to String#gsub! (or
gsub in the frozen case).

CHANGELOG

@@ -6,8 +6,6 @@
 
 * Support :literal_prefix and :literal_postfix options for how to output literal tags (e.g. <%% code %>) (jaredcwhite) (#26, #27)
 
-* Reduce memory allocation during template parsing (fatkodima, jeremyevans) (#25)
-
 === 1.9.0 (2019-09-25)
 
 * Change default :bufvar from 'String.new' to '::String.new' to work with BasicObject (jeremyevans)

lib/erubi.rb

@@ -179,12 +179,10 @@ def initialize(input, properties={})
     # Add raw text to the template.  Modifies argument if argument is mutable as a memory optimization.
     def add_text(text)
       if text && !text.empty?
-        include_slash = text.include?('\\')
-        include_apos = text.include?("'")
-        if include_slash || include_apos
-          text = text.dup if text.frozen?
-          text['\\'] = '\\\\' if include_slash
-          text["'"] = "\\'" if include_apos
+        if text.frozen?
+          text = text.gsub(/['\\]/, '\\\\\&')
+        else
+          text.gsub!(/['\\]/, '\\\\\&')
         end
         @src << " " << @bufvar << " << '" << text << TEXT_END
       end

test/test.rb

@@ -121,6 +121,49 @@ def self.quux
 END3
   end
 
+  it "should escape all backslashes and apostrophes in text" do
+    list = ['&\'<>"2']
+    check_output(<<END1, <<END2, <<END3){}
+<table>
+ <tbody>' ' \\ \\
+  <% i = 0
+     list.each_with_index do |item, i| %>
+  <tr>
+   <td><%= i+1 %></td>
+   <td><%== item %></td>
+  </tr>
+ <% end %>
+ </tbody>
+</table>
+<%== i+1 %>
+END1
+_buf = ::String.new; _buf << '<table>
+ <tbody>\\' \\' \\\\ \\\\
+';   i = 0
+     list.each_with_index do |item, i| 
+ _buf << '  <tr>
+   <td>'; _buf << ( i+1 ).to_s; _buf << '</td>
+   <td>'; _buf << ::Erubi.h(( item )); _buf << '</td>
+  </tr>
+';  end 
+ _buf << ' </tbody>
+</table>
+'; _buf << ::Erubi.h(( i+1 )); _buf << '
+';
+_buf.to_s
+END2
+<table>
+ <tbody>' ' \\ \\
+  <tr>
+   <td>1</td>
+   <td>&amp;&#39;&lt;&gt;&quot;2</td>
+  </tr>
+ </tbody>
+</table>
+1
+END3
+  end
+
   it "should strip only whitespace for <%, <%- and <%# tags" do
     check_output(<<END1, <<END2, <<END3){}
   <% 1 %>