Make sure to strip "/" for static files if URIPrefix ends with a "/"

[gdey]

When serving up static files we need to strip any proxy prefixes we may add onto the URL.
We forgot to check to see if the prefix had a "/" at the end of it, and if it did made sure
to strip it out, as static files should always start with a "/". However, if we did not remove
the "/" at the end of the prefix that we then use to trim the url; we would end up trimming off
the starting "/". This fixes that.

Fixes #735

[gdey]

Have not had a chance to test this yet.

[coveralls]

Pull Request Test Coverage Report for Build 8e011cb69-PR-771

• 0 of 9 (0.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 45.392%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
server/viewer.go	0	9	0.0%

Files with Coverage Reduction	New Missed Lines	%
server/viewer.go	1	40.0%

Totals
Change from base Build 7908653b9:	-0.03%
Covered Lines:	5413
Relevant Lines:	11925

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💛 - Coveralls