Setting context attributes for logging handler

[janhaviagrawal]

With the latest release, it is possible to set up context.properties for logging handler but it still does not allow setting other context attributes (user, operation etc) passed as channel.context.
This is because in the channel write code

local_context = context or self._context
for key, value in self._write_tags(local_context):
    tags[key] = value

This ignores the attributes set in the channel itself. This should be

for prop_context in [self._context, context]:
    for key, value in self._write_tags(prop_context):
        tags[key] = value

[janhaviagrawal]

PR for this: #114

[SergeyKanzhelev]

Thanks again for contribution. As I mentioned in PR - please send CHANGELOG update as well. Closing issue

[janhaviagrawal]

PR for CHANGELOG #116