Making ActiveRecord #merge_joins strip whitespace

In edge Rails the joins are nicely merged and duplicate join strings are uniq'ed out. However, if there is still some whitespace at the start and/or beginning, which is the case when hash-style joins are converted to string joins, they won't be an exact string match and will thus duplicates will not be removed and an ActiveRecord::InvalidStatement will most likely be raised.

Of course other things, such as database specific escaping, can still cause identical joins to not be an exact string match, however this patch will at least improve the chance of matches since it strips all joins of leading/trailing whitespaces.