Metamorph instantiation exception when using "urlencode"

[dr0i]

Fixes #705.

This prevents a Metamorph instantiation exception.

[dr0i]

1. functional review: @TobiasNx
2. if 1) is fine: code review: @blackwinter

[blackwinter]

It's nice that you added a test for urlencode (the first one!). You might also go one step further and test this option explicitly (plusforspace="false").

[TobiasNx]

I tested

<?xml version="1.0" encoding="UTF-8"?>
<metamorph xmlns="http://www.culturegraph.org/metamorph" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	version="1">
	<rules>
		<data source="_id" name="id"/>
		<data source="a" name="title">
			<urencode/>
		</data>
	</rules>
</metamorph>

<?xml version="1.0" encoding="UTF-8"?>
<metamorph xmlns="http://www.culturegraph.org/metamorph" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	version="1">
	<rules>
		<data source="_id" name="id"/>
		<data source="a" name="title">
			<urlencode  plusforspace="false"/>
		</data>
	</rules>
</metamorph>

uri_encode("test", plus_for_space:"false", safe_chars:"")

All worked! ✔️