Fixes Issue 1252

Handle CAPABILITY response in following cases:
1) When sent as part of the banner, no longer sends explicit
CAPABILITY command

2) When sent as an UNTAGGED response to LOGIN, updates stored
   capabilities.

3) When sent as part of the TAGGED response to LOGIN, updates stored
   capabilities.

Due to odd implementation of the CRAM-MD5 authentication, the
capability updates will not happen as part of the CRAM-MD5
authentication.