Avoid calling String#strip on invalid Strings

Use case: some servers have been known to serve files with
Content-Types like “video/mp4; charset=utf-8”, which HTTParty dutifully
tells ruby is a UTF-8 string.  However, before the Parser can decide
that MP4 is not a format it is capable of parsing, the response body is
tested for emptiness … after whitespace is stripped.

Since the String isn’t actually a valid UTF-8 String, this raises an
ArgumentError.  This patch fixes that error by avoiding the call on
Strings that are not validly encoded.

lib/httparty/parser.rb

@@ -98,7 +98,9 @@ def initialize(body, format)
     # @return [Object] the parsed body
     # @return [nil] when the response body is nil, an empty string, spaces only or "null"
     def parse
-      return nil if body.nil? || body.strip.empty? || body == "null"
+      return nil if body.nil?
+      return nil if body == "null"
+      return nil if body.valid_encoding? && body.strip.empty?
       if supports_format?
         parse_supported_format
       else

spec/httparty/parser_spec.rb

@@ -97,6 +97,12 @@ class MyParser < HTTParty::Parser
       allow(@parser).to receive_messages(body: "   ")
       expect(@parser.parse).to be_nil
     end
+
+    it "does not raise exceptions for bodies with invalid encodings" do
+      allow(@parser).to receive_messages(body: "\x80")
+      allow(@parser).to receive_messages(supports_format?: false)
+      expect(@parser.parse).to_not be_nil
+    end
   end
 
   describe "#supports_format?" do