Content-type case-conversion changes.

Don't convert the content-type to lower case in
MimeMessage.getContentType.  The content-type may have optional parameters
that are case sensitive (boundary, name).

In removing the lower-case conversion from getContentType, a review was
made for inappropriate case-sensitive comparisons which use data obtained
with getContentType.  The only ones found were in isMimeType in both
Message and MimeBodyPart.

Case-sensitive instances of isMimeType were made case-insensitive.  Also,
isMimeType was moved from Message to MimeMessage for symmetry with
MimeBodyPart (MimeMessage & MimeBodyPart are similar and contain a good
bit of duplication such as this).

The unit test required fixing now that the case of the boundary text is
preserved.

References:

Commits 2c5186 and dc4002 added the toLowerCase to getContentType in
MimeMessage & MimeBodyPart (Issue 94).

Later, commit 50cd60 removed the toLowerCase addition from MimeBodyPart
(Issue 1289).