Use Ruby 2.3's +'' instead of String.new
Use Ruby 2.3's +'' instead of String.new

This is another Rubocop performance cop. Here's their description:

```
In Ruby 2.3 or later, use unary plus operator to unfreeze a string literal instead of String#dup and String.new. Unary plus operator is faster than String#dup.

Note: String.new (without operator) is not exactly the same as +''. These differ in encoding. String.new.encoding is always ASCII-8BIT. However, (+'').encoding is the same as script encoding(e.g. UTF-8). So, if you expect ASCII-8BIT encoding, disable this cop.
```

Signed-off-by: Tim Smith <764ef62106582a09ed09dfa0b6bff7c05fd7d1e4@chef.io>

chef/chef,chef/chef,tomdoherty/chef,jaymzh/chef,higanworks/chef,martinisoft/chef,Ppjet6/chef,martinisoft/chef,MsysTechnologiesllc/chef,nvwls/chef,martinisoft/chef,jaymzh/chef,nvwls/chef,mwrock/chef,tas50/chef-1,martinisoft/chef,tas50/chef-1,tomdoherty/chef,tomdoherty/chef,gene1wood/chef,juliandunn/chef,juliandunn/chef,Tensibai/chef,MsysTechnologiesllc/chef,martinisoft/chef,juliandunn/chef,chef/chef,MsysTechnologiesllc/chef,Ppjet6/chef,MsysTechnologiesllc/chef,juliandunn/chef,mwrock/chef,tomdoherty/chef,jaymzh/chef,chef/chef,mwrock/chef,Ppjet6/chef,nvwls/chef,mattray/chef,mattray/chef,Ppjet6/chef,Tensibai/chef,mattray/chef,Ppjet6/chef,mwrock/chef,Ppjet6/chef,Tensibai/chef,tas50/chef-1,Tensibai/chef,mattray/chef,higanworks/chef,nvwls/chef,nvwls/chef,higanworks/chef,mattray/chef,gene1wood/chef,tomdoherty/chef,higanworks/chef,gene1wood/chef,Tensibai/chef,nvwls/chef,gene1wood/chef,juliandunn/chef,jaymzh/chef,higanworks/chef,tas50/chef-1