force to use utf-8 for template processing


When non-ascii characters like CJK are given in data to template, following error is occurred:

	/.../ruby-docx-templater/lib/docx_templater/template_processor.rb:19:in `gsub!': incompatible character encodings: ASCII-8BIT and UTF-8 (Encoding::CompatibilityError)
	        from /.../ruby-docx-templater/lib/docx_templater/template_processor.rb:19:in `block in render'
	        from /.../ruby-docx-templater/lib/docx_templater/template_processor.rb:14:in `each'
	        from /.../ruby-docx-templater/lib/docx_templater/template_processor.rb:14:in `render'
	        from /.../ruby-docx-templater/lib/docx_templater/docx_creator.rb:30:in `copy_or_template'
	        from /.../ruby-docx-templater/lib/docx_templater/docx_creator.rb:46:in `block (3 levels) in create_new_zip_in_memory'
	        from /.../ruby-docx-templater/lib/docx_templater/docx_creator.rb:45:in `fopen'
	        from /.../ruby-docx-templater/lib/docx_templater/docx_creator.rb:45:in `block (2 levels) in create_new_zip_in_memory'
	        from /.../ruby-docx-templater/lib/docx_templater/docx_creator.rb:43:in `times'
	        from /.../ruby-docx-templater/lib/docx_templater/docx_creator.rb:43:in `block in create_new_zip_in_memory'
	        from /.../ruby-docx-templater/lib/docx_templater/docx_creator.rb:42:in `open_buffer'
	        from /.../ruby-docx-templater/lib/docx_templater/docx_creator.rb:42:in `create_new_zip_in_memory'
	        from /.../ruby-docx-templater/lib/docx_templater/docx_creator.rb:21:in `block in generate_docx_bytes'
	        from /.../ruby-docx-templater/lib/docx_templater/docx_creator.rb:36:in `block in read_existing_template_docx'
	        from /.../ruby-docx-templater/lib/docx_templater/docx_creator.rb:35:in `open'
	        from /.../ruby-docx-templater/lib/docx_templater/docx_creator.rb:35:in `read_existing_template_docx'
	        from /.../ruby-docx-templater/lib/docx_templater/docx_creator.rb:20:in `generate_docx_bytes'
	        from /.../ruby-docx-templater/lib/docx_templater/docx_creator.rb:14:in `generate_docx_file'
	        from test.rb:18:in `<main>'

This pull request fixes this problem by using utf-8 encoding by default.
This change may work well for other documents since almost DOCXs are utf-8.

I am a newby of Ruby so please use better solution if you have.