Another Ruby 2.6 BigDecimal compatibility issue

This patch modifies XmlMini::Parsing["decimal"] to handle a string that
contains an invalid number. Since [ruby/ruby@a0e438c#diff-6b866d482baf2bdfd8433893fb1f6d36R144](https://github.com/ruby/ruby/commit/a0e438cd3c28d2eaf4efa18243d5b6edafa14d88#diff-6b866d482baf2bdfd8433893fb1f6d36R144) this case raises an `ArgumentError`. `String.to_f` returns 0.0 if there is not a valid number at the start of the argument, so current behavior is conserved.

See https://travis-ci.org/rails/rails/jobs/463180341#L6264

Related: #34600, #34601

activesupport/lib/active_support/xml_mini.rb

@@ -71,7 +71,7 @@ def content_type
             begin
               BigDecimal(number)
             rescue ArgumentError
-              BigDecimal("0")
+              BigDecimal(number.to_f.to_s)
             end
           else
             BigDecimal(number)