Access to context fails inside nested serializer

[jtrain]

As discussed in #5 a DynamicFieldMixin-ed serializer that accesses context will fail when used as a nested serializer.

I found this issue when I was re-using a serializer as a nested serializer, and that serializer relies on context['request'] to build a url for one of its fields.

Without fixing drf-dynamic-fields to allow nested serializers, there are a number of work arounds:

1. You could just have two separate serializers, one for nesting and one for normal views.
2. The nested serializer can manually find the correct context by chasing the parent fields up the tree until it finds the one with context set.

I used 2) for a while before deciding it was better to fix the underlying issue.

[jtrain]

#8 This one has the PR now for this.

[dbrgn]

Fixed by #10 I think.