(MODULES-444) Add specs for new behaviour

`concat` can now take multiple arguments
(MODULES-444) Acceptance test for primitives

`concat` should be able to concat arrays and primitives
(MODULES-444) Acceptance for multiple arrays

Acceptance test to take multiple arrays for concatenation
(MODULES-444) Change test to > 2 arguments

Also add extra test for just 1 argument
(MODULES-444) - Check for accepting > 2 args
(MODULES-444) Update docs with new functionality
(MODULES-444) Change argument restriction to < 2
(MODULES-444) - Real meat of the change

This is the core change, we now go through the array and add it to the first element, instead of just two arguments.