Fix Shorthand linter false positives

For properties with names beginning with a shorthandable property (like
`border-color` or `margin`) followed by interpolation, the `Shorthand`
linter would report a lint if that property had values that were not in
the most condensed form.

These lints were erroneous because since the lint was basically being
triggered due to the prefix of the property name. Since the
interpolation cannot be determined statically, it's possible that it
would resolve to a property that did not support shorthand values.

Fix the bug by checking the entire property name, not just the prefix.

In the process the code was cleaned up a bit, since Code Climate
indicated that the `visit_prop` method had a high cyclomatic complexity
due to the nesting in the case statement.

Change-Id: Ia15b0f9feb1ad050300f243548907a957341e302
Reviewed-on: https://gerrit.causes.com/29017
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>