[TACHYON-240] bug fix for new join test in TachyonURITest

the last assert in the new added tests in TachyonURITest will fail without current modification in TachyonURI because
 `new TachyonURI("/").join(pathWithSpecialCharAndColon));`
will cause
 `new TachyonURI("/" + TachyonURI.SEPARATOR + pathWithSpecialCharAndColon);`
in method `join`
so the final input to TachyonURI constructor is "//host:port", the joined path will be interpreted as authority component by TachyonURI constructor. in our new test, the host part contains illegal characters, then the bug described in https://tachyon.atlassian.net/browse/TACHYON-240 is triggered.