Skip multiple (bogus) colons after header field name

Some (broken) implementation sends double-colons for certain fields,
and this change makes vmime support that case.

src/vmime/headerField.cpp

@@ -135,6 +135,7 @@ shared_ptr <headerField> headerField::parseNext
 				                  buffer.begin() + nameEnd);
 
 				// Skip ':' character
+				while (pos < end && buffer[pos] == ':')
 					++pos;
 
 				// Skip spaces between ':' and the field contents