Clarify that contexts are invariant during instance data processing #638
Description
A context, especially in 1.1, might have multiple references to the same secondary context. It should be clear in the normative text that these references are invariant -- that once imported the first time, that exact representation must be used for all other references to it. Otherwise if the context representation changes during processing, the resulting data will be internally inconsistent.
In particular, the context processing algorithm says:
If context is a string, it represents a reference to a remote context. We dereference the remote context and replace context with the value of the @context key of the top-level object in the retrieved JSON-LD document.
And doesn't mention reuse of any context cached against that URI.