Fix form field ordering when backed by Hash nodes

When a form field is created with a Hash as the node instead of a
nokogiri node, the sort order was previously undefined.  This can happen
when a user calls `Mechanize.post` or when a Form is manually created
with Hash backed Fields and the user calls `Mechanize.submit` or
`Mechanize::Form#submit`.

fixes #307
Merge branch 'fix-field-order' of https://github.com/ryansch/mechanize into ryansch-fix-field-order

Conflicts:
	lib/mechanize/form/field.rb