Fix `frozen dict` error in project SLAs

Writing changes to documents could result in an error in methods
writing on the current context. In fact, in v8 context is a frozen dict
and is not writable.
As a workaround, the methods now use a `ctx` dict copy of `context`.
Merge pull request #38 from dreispt/fix-sla

Fix `frozen dict` error in project SLAs