Unicodify the return value of TwillTestCase.last_page()

inside the check_page_for_string() method.

Fix the following stacktrace during ToolShed tests:

```
Traceback (most recent call last):
  File "/usr/lib/python2.7/unittest/case.py", line 329, in run
    testMethod()
  File "/galaxy/test/shed_functional/functional/test_1160_tool_help_images.py", line 79, in test_0010_load_tool_page
    self.load_display_tool_page(repository, tool_path, changeset_revision, strings_displayed=[image_path], strings_not_displayed=[])
  File "/galaxy/test/shed_functional/base/twilltestcase.py", line 1122, in load_display_tool_page
    self.check_for_strings(strings_displayed, strings_not_displayed)
  File "/galaxy/test/functional/twilltestcase.py", line 39, in check_for_strings
    self.check_page_for_string(string)
  File "/galaxy/test/functional/twilltestcase.py", line 69, in check_page_for_string
    if page.find(patt) == -1:
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 8760: ordinal not in range(128)
```

test/functional/twilltestcase.py

@@ -16,6 +16,7 @@
 from twill.other_packages._mechanize_dist import ClientForm
 
 from base.testcase import FunctionalTestCase  # noqa: I100,I202
+from galaxy.util import unicodify  # noqa: I201
 
 # Force twill to log to a buffer -- FIXME: Should this go to stdout and be captured by nose?
 buffer = StringIO()
@@ -65,7 +66,7 @@ def check_page(self, strings_displayed, strings_displayed_count, strings_not_dis
 
     def check_page_for_string(self, patt):
         """Looks for 'patt' in the current browser page"""
-        page = self.last_page()
+        page = unicodify(self.last_page())
         if page.find(patt) == -1:
             fname = self.write_temp_file(page)
             errmsg = "no match to '%s'\npage content written to '%s'\npage: [[%s]]" % (patt, fname, page)
@@ -205,6 +206,10 @@ def json_from_url(self, url, params={}):
         return loads(self.last_page())
 
     def last_page(self):
+        """
+        Return the last visited page (usually HTML, but can binary data as
+        well).
+        """
         return tc.browser.get_html()
 
     def last_url(self):