Change +'' to ''.dup for backwards compat with older versions of ruby

See #733

Author: jnunemaker

lib/httparty/hash_conversions.rb

@@ -28,7 +28,7 @@ def self.to_params(hash)
     def self.normalize_param(key, value)
       normalized_keys = normalize_keys(key, value)
 
-      normalized_keys.flatten.each_slice(2).inject(+'') do |string, (k, v)|
+      normalized_keys.flatten.each_slice(2).inject(''.dup) do |string, (k, v)|
         string << "#{ERB::Util.url_encode(k)}=#{ERB::Util.url_encode(v.to_s)}&"
       end
     end

lib/httparty/request/body.rb

@@ -35,7 +35,7 @@ def multipart?
       def generate_multipart
         normalized_params = params.flat_map { |key, value| HashConversions.normalize_keys(key, value) }
 
-        multipart = normalized_params.inject(+'') do |memo, (key, value)|
+        multipart = normalized_params.inject(''.dup) do |memo, (key, value)|
           memo << "--#{boundary}#{NEWLINE}"
           memo << %(Content-Disposition: form-data; name="#{key}")
           # value.path is used to support ActionDispatch::Http::UploadedFile