Fix filenames of two function spec tests

The tests weren't being run.  Total tests increase from 2742 to 2769.

Also fix 'when using a class extending String' test.
It had been failing with...
```
RuntimeError:
       can't modify frozen AlsoString
```

Author: alexjfisher

lib/puppet/functions/length.rb

@@ -3,7 +3,7 @@
   dispatch :length do
     param 'Variant[String,Array,Hash]', :value
   end
-  def	length(value)
+  def length(value)
     if value.is_a?(String)
       result = value.length
     elsif value.is_a?(Array) || value.is_a?(Hash)

spec/functions/is_function_available.rb renamed to spec/functions/is_function_available_spec.rb

@@ -26,10 +26,6 @@
   it { is_expected.to run.with_params('āβćđ').and_return(4) }
 
   context 'when using a class extending String' do
-    it 'should call its size method' do
-      value = AlsoString.new('asdfghjkl')
-      value.expects(:length).returns('foo')
-      expect(subject).to run.with_params(value).and_return('foo')
-    end
+    it { is_expected.to run.with_params(AlsoString.new('asdfghjkl')).and_return(9) }
   end
 end