Jvmargs should not modify its arguments

Side effects lead to weird behavior when they leak out of a class. I found that if I did something like this in Chef:

```ruby
#assume node[:tomcat][:java_options] = "-Xmx1024M -XX:MaxPermSize=256M"
args = JVMArgs::Args.new(node[:tomcat][:java_options])
```

then when I looked at `node[:tomcat][:java_options]` later (e.g. when filling in a template), it was changed to `"Xmx1024M -XX:MaxPermSize=256M"`, which broke tomcat. As I said, unexpected side effects are bad, so I fixed them.