Fix safe Style/ColonMethodCall offenses

Caxlsx is using both `.` and `::`, 220 occurrences vs 280 to invoke
methods on `Axlsx` module.

This commit standardizes the approach towards `.`, which will also allow
shorter lines.

Performance is not affected

```
Comparison:
      Axlsx.validate:  8515252.3 i/s
     Axlsx::validate:  8512863.7 i/s - same-ish: difference falls within error 
```

1 files changed, 3 insertions, 3 deletions

diff --git a/test/util/tc_mime_type_utils.rb b/test/util/tc_mime_type_utils.rb
index 16ba45cd..f990e8e2 100644
--- a/test/util/tc_mime_type_utils.rb
+++ b/test/util/tc_mime_type_utils.rb
@@ -14,11 +14,11 @@ class TestMimeTypeUtils < Test::Unit::TestCase
   def teardown; end
 
   def test_mime_type_utils
-    assert_equal('image/jpeg', Axlsx::MimeTypeUtils::get_mime_type(@test_img))
-    assert_equal('image/png', Axlsx::MimeTypeUtils::get_mime_type_from_uri(@test_img_url))
+    assert_equal('image/jpeg', Axlsx::MimeTypeUtils.get_mime_type(@test_img))
+    assert_equal('image/png', Axlsx::MimeTypeUtils.get_mime_type_from_uri(@test_img_url))
   end
 
   def test_escape_uri
-    assert_raise(URI::InvalidURIError) { Axlsx::MimeTypeUtils::get_mime_type_from_uri('| ls') }
+    assert_raise(URI::InvalidURIError) { Axlsx::MimeTypeUtils.get_mime_type_from_uri('| ls') }
   end
 end