Perform case insensitive comparisons for T.38 attributes

Perform case insensitive comparisons for T.38 attributes

RFC5347 section 2.5.2 states the following:
...
The attribute "T38MaxBitRate" was once incorrectly registered with
IANA as "T38maxBitRate" (lower-case "m"). In accordance with T.38
examples and common implementation practice, the form "T38MaxBitRate"
SHOULD be generated by implementations conforming to this package.
In general, it is RECOMMENDED that implementations of this package
accept lowercase, uppercase, and mixed upper/lowercase encodings of
all the T.38 attributes.
...

Asterisk currently does not perform case insensitive matching on the T.38
attributes. This causes the T38MaxBitRate attribute to be negotiated at
2400 baud instead of 14400 (or whatever value you actually wanted).

This patch makes it so that when we compare T.38 attributes, we do so in a case
insensitive fashion.

Note that while the issue reporter did not directly write the patch, they
contributed to it (and would have provided one themselves if the license had
gone through a tad faster), and hence get attribution for it.

Review: https://reviewboard.asterisk.org/r/2298/

(closes issue ASTERISK-20897)
Reported by: Eric Hill
Tested by: Eric Hill
patches:
  -- uploaded by Eric Hill
........

Merged revisions 380458 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@380465 65c4cc65-6c06-0410-ace0-fbb531ad65f3