Fix for applyOptionsWithDefault not keeping unmodified options

[sgtsquiggs]

I assume this was just an oversight. I was expecting that any specified options would override defaults, but unspecified options would still use the default options.

IE:

// Given
defaultOptions := &Options{
	expiration: 25 * time.Second,
}

// When
options := applyOptionsWithDefault(defaultOptions, WithCost(7))

// Then
assert.Equal(t, int64(7), options.cost)
assert.Equal(t, 25*time.Second, options.expiration)

[sgtsquiggs]

fixes #168

[sgtsquiggs]

failures seem unrelated

[eko]

Yes, I will take a look later. Thank you for this PR!