Update HTTPHeaderMap wrappers to use AtomicString type for header values

Source/wtf/text/StringOperators.h

Index: Source/wtf/text/StringOperators.h
diff --git a/Source/wtf/text/StringOperators.h b/Source/wtf/text/StringOperators.h
index 0a8ee93e3d8f5783c6e122b87eccabe340abfdb0..a4ed953d3c86e2ffa2043d62172cecac8e46ee19 100644
--- a/Source/wtf/text/StringOperators.h
+++ b/Source/wtf/text/StringOperators.h
@@ -35,13 +35,12 @@ public:
 
     operator String() const
     {
-        RefPtr<StringImpl> resultImpl = makeString(m_string1, m_string2);
-        return resultImpl.release();
+        return String(makeString(m_string1, m_string2));
     }
 
     operator AtomicString() const
     {
-        return operator String();
+        return AtomicString(makeString(m_string1, m_string2));
     }
 
     bool is8Bit()