Prevent mutation of notifier arguments

[nepalez]

Method GELF::Notifier.stringify_keys mutates its arguments which leads to mutating of arguments of the #notify invocations.

This behaviour is unsafe, and tbh not necessary, because the method returns processed hash, that could be a new object.

Previous behavior:

data = { foo: { bar: "BAZ" } }

n = GELF::Notifier.new("localhost", 12201)
n.notify!(:short_message => "foo", :full_message => "something here\n\nbacktrace?!", data)

data # => { "foo" => { "bar" => "BAZ" } } # keys are stringified in the source hash

The new one:

data = { foo: { bar: "BAZ" } }

n = GELF::Notifier.new("localhost", 12201)
n.notify!(:short_message => "foo", :full_message => "something here\n\nbacktrace?!", data)

data # => { foo: { bar: "BAZ" } } # the source hash not changed

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