stringifyQuery support multiple fields

[kuserich]

stringifyQuery currently does not support multiple fields in the input object.

stringifyQuery( [ {
    product: 1885,
    action: 'hook_name',
} ] );

returns

'product,action=1885,hook_name'

should return

'product=1885,action=hook_name'

This issue was first raised in a discussion in #76