pom.xml(maven-enforcer-plugin): updated to 1.3.1

@see http://mail-archives.apache.org/mod_mbox/maven-announce/201307.mbox/%3Cop.w0fqerd7kdkhrr@columbia%3E

No functional changes.
pom.xml: updated H2 to 1.3.174

@see http://www.h2database.com/html/changelog.html

No functional changes.
ImageServiceImpl: improved assertion message.

No functional changes.
ImageServiceImpl: handle Content-Type with charset correctly.

Found during updating Selenium to >= 2.36.0 which adds parameter with charset to Content-Type header
(even for images).
pom.xml: updated elenium to 2.37.1

@see http://selenium.googlecode.com/git/java/CHANGELOG

No functional changes.