952735: Ensure that prefixes plus content urls do not result in double slashes
952735: Add additional checks with content prefixes with many trailing / and content urls with many leading /
ensure null/empty contentPrefix handled. remove endless loop.

Since we already were using StringUtils for this class, I simply
reused it again to create the full content path. The idea here
is to strip the / from the end of the contentprefix and from the
beginning of the content path. Then use a single prefix.
Merge pull request #229 from candlepin/bkearney/952735

952735: Ensure that prefixes plus content urls do not result in double s...