Fix FrozenString error

An error was apparent in specs when trying to assign a string as the price. It's not a problem when submitting the form in the browser, I don't know why.

But in any case, it shouldn't be trying to modify a variable passed as a parameter.

app/models/spree/price.rb

@@ -44,7 +44,7 @@ module Spree
                                       :'number.currency.format.delimiter'])
       non_price_characters = /[^0-9\-#{separator}]/
       # Strip everything else first
-      price.gsub!(non_price_characters, '')
+      price = price.gsub(non_price_characters, '')
       # Then replace the locale-specific decimal separator with the standard separator if necessary
       price.gsub!(separator, '.') unless separator == '.'
 

spec/models/spree/price_spec.rb

@@ -25,5 +25,14 @@ module Spree
         expect(expensive_variant.persisted?).to be true
       end
     end
+
+    context "with string" do
+      it "parses the price" do
+        variant.price = " 10.25 eur"
+        variant.save
+
+        expect(variant.reload.price).to eq 10.25
+      end
+    end
   end
 end