Base64 tests extending

* Base64 tests extended, now contain test of alternative alphabets
* Test Base64BTest renamed because all variants are now tested in one test
* `Base64.calculateDecodedLength()` made public after discussing with fjuma (user needs this method to prepare output byte array for non-standard alphabets)
The suggested change (`if (len < 2)`) sounds good.  As for guessing what is more optimizable by the compiler - don't; compiler writers are very clever and the compiler can generally analyze the assignments pretty well.  Unless you have a JMH microbenchmarks proving that there's a real problem, don't try to micro-optimize like this.