add an instance cache for is(something) calls
remove isCache method

except for mobile, no is calls should persist. So it's not likely to be
of any use
move the normalizing logic to the public function
fix almost all tests
add a function for clearing the detector cache
better test description

This appears to be the actual intent of this test
add parentheses to referenced method name

prevents potential confusion
Merge pull request #5049 from AD7six/3.0-cache-is-calls

cache is calls