Fix for test_predicates_generic_by_label: generate a better script method call for labels

Fixes the following exception:
Traceback (most recent call last):
File "/home/travis/build/roignac/dogtail/tests/test_predicate.py", line 160, in test_predicates_generic_by_label
  False), u'child(label="dummy name 1", recursive=False)')
  AssertionError: u'child(label=\'"dummy name 1"\', recursive=False)' != u'child(label="dummy name 1", recursive=False)'
  - child(label='"dummy name 1"', recursive=False)
 ? - -
 + child(label="dummy name 1", recursive=False)