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/workbook/worksheet/tc_worksheet.rb b/test/workbook/worksheet/tc_worksheet.rb
index 4ec74efb..464d2e64 100644
--- a/test/workbook/worksheet/tc_worksheet.rb
+++ b/test/workbook/worksheet/tc_worksheet.rb
@@ -456,14 +456,14 @@ class TestWorksheet < Test::Unit::TestCase
   end
 
   def test_to_xml_string_with_illegal_chars
-    old = Axlsx::trust_input
-    Axlsx::trust_input = false
+    old = Axlsx.trust_input
+    Axlsx.trust_input = false
     nasties = "\v\u2028\u0001\u0002\u0003\u0004\u0005\u0006\u0007\u0008\u001f"
     @ws.add_row [nasties]
 
     assert_equal(0, @ws.rows.last.cells.last.value.index("\v"))
     assert_nil(@ws.to_xml_string.index("\v"))
-    Axlsx::trust_input = old
+    Axlsx.trust_input = old
   end
 
   def test_to_xml_string_with_newlines