fix contrib check-mime-type.pl for changes to 'svnlook proplist' output

Since r1416637 (released in 1.7.8), the changed output of svnlook proplist from name : value format to a new multi-line/multi-value format made the existing check-mime-type.pl contrib pre-commit hook script no longer worked correctly, as it could no longer parse the multi-line proplist output.

First mentioned on the mailing list back in March 2014:
http://mail-archives.apache.org/mod_mbox/subversion-dev/201403.mbox/%3C1576503.m6XB7udPXQ@hurry.speechfxinc.com%3E

New thread for this:
http://mail-archives.apache.org/mod_mbox/subversion-dev/201508.mbox%3CCALeEUB7iz6u7LLYm09+dkOSJb3+ZsjOGMpHg28DmttjyxQiVOw@mail.gmail.com%3E

New patch proposed:
http://mail-archives.apache.org/mod_mbox/subversion-dev/201508.mbox/%3C1440376950-3727-1-git-send-email-javabrett%40gmail.com%3E

http://mail-archives.apache.org/mod_mbox/subversion-dev/201508.mbox/%3C1440376950-3727-2-git-send-email-javabrett%40gmail.com%3E

Original issue reported by javabrett