Fix: #746 Compile error under VS2015 (#747)

Co-authored-by: jigarcia@vaxtor.es <jigarcia@vaxtor.es>

httplib.h

@@ -2979,9 +2979,8 @@ public:
 
   bool is_valid() const { return is_valid_; }
 
-  template <typename T, typename U>
-  bool parse(const char *buf, size_t n, const T &content_callback,
-             const U &header_callback) {
+  bool parse(const char *buf, size_t n, const ContentReceiver &content_callback,
+	  const MultipartContentHeader &header_callback) {
 
     static const std::regex re_content_disposition(
         "^Content-Disposition:\\s*form-data;\\s*name=\"(.*?)\"(?:;\\s*filename="