Ignore extra headers in MSRP

[jasonshugart]

Summary
This PR fixes an issue if MSRP messages container other/non-required headers.

Details
If an MSRP message contains a header such as Content-Disposition, which is a recognized header, the message will get rejected with "ERROR:core:handle_mi_request: Invalid parameters". This is a valid header in an MSRP message as described in RFC 4975, section 9.

Solution
This fix moves the "default" switch case to be grouped with HDR_OTHER_T instead of HDR_ERROR_T and allows for the continued processing of the message.

Compatibility
This should not cause any issues with existing code unless it was relying on a message with other headers being rejected.

Closing issues

[bogdan-iancu]

Hi @jasonshugart , I would rather allow Content-Disposition hdr, rather than allowing all. Just to be able to keep trace of what is ok to be used or not in MSRP stuff.

[jasonshugart]

@bogdan-iancu There are several other headers that are valid, such as Content-Description, Content-ID, and others defined in RFC2045. Restricting this to only allow for specific headers will eventually break future MSRP message parsing, especially when we start handling RCS over MSRP.