Attatches a MIME field to a header. The header is represented by the bufp and hdr
arguments which should have been obtained by a call to
TSHttpTxnClientReqGet() or similar. If
the field in field was created by calling
TSMimeHdrFieldCreateNamed() the same
bufp and hdr passed to that should be passed to this function.
TS_SUCCESS if the field was attached to the header,
TS_ERROR if it
was not. Fields cannot be attached to read only headers.