folly: operator << for MutableStringPiece

Test Plan: n.a

Reviewed By: philipp@fb.com

FB internal diff: D1487919

folly/Range.cpp

@@ -37,6 +37,11 @@ std::ostream& operator<<(std::ostream& os, const StringPiece& piece) {
   return os;
 }
 
+std::ostream& operator<<(std::ostream& os, const MutableStringPiece& piece) {
+  os.write(piece.start(), piece.size());
+  return os;
+}
+
 namespace detail {
 
 size_t qfind_first_byte_of_memchr(const StringPiece& haystack,

folly/Range.h

@@ -804,6 +804,7 @@ typedef Range<const unsigned char*> ByteRange;
 typedef Range<unsigned char*> MutableByteRange;
 
 std::ostream& operator<<(std::ostream& os, const StringPiece& piece);
+std::ostream& operator<<(std::ostream& os, const MutableStringPiece& piece);
 
 /**
  * Templated comparison operators