Fix parseBold function if we have multiple bold strings

[peti2001]

By default the preg_replace function is un-greedy. I used the U flag to make it greedy. I added a new test case to cover this case.

I was not sure, how do you want to handle this case. Should we put two asserts in one test or write two separated test cases. Or add the testDoubleBoldTextCanBeParsed test first and fix it in another part.