can't modify frozen String when run with --enable=frozen-string-literal flag

[khiav223577]

My rails project, which is running with RUBYOPT="--enable-frozen-string-literal", encounter this problem.
The Error message is:

Failure/Error: package.serialize(tempfile)

     FrozenError:
       can't modify frozen String: ""
     # /home/khiav/.rvm/gems/ruby-3.0.0/gems/axlsx-2.0.1/lib/axlsx/rels/relationships.rb:23:in `to_xml_string'
     # /home/khiav/.rvm/gems/ruby-3.0.0/gems/axlsx-2.0.1/lib/axlsx/package.rb:197:in `parts'
     # /home/khiav/.rvm/gems/ruby-3.0.0/gems/axlsx-2.0.1/lib/axlsx/package.rb:160:in `write_parts'
     # /home/khiav/.rvm/gems/ruby-3.0.0/gems/axlsx-2.0.1/lib/axlsx/package.rb:105:in `block in serialize'
     # /home/khiav/.rvm/gems/ruby-3.0.0/gems/rubyzip-1.0.0/lib/zip/output_stream.rb:49:in `open'
     # /home/khiav/.rvm/gems/ruby-3.0.0/gems/axlsx-2.0.1/lib/axlsx/package.rb:104:in `serialize'

It looks like the default arguments of to_xml_string method is a frozen string when running with the flag.

axlsx/lib/axlsx/rels/relationships.rb

Line 24 in 83e2398

def to_xml_string(str = '')

I suggest we replace '' with String.new, so that the empty string will never be frozen.

[noniq]

Just for the record: This has already been fixed in caxlsx, see caxlsx/caxlsx@fabe8cb Not yet released though, will probably be in v4.0.0