Client Logging Interceptor defaults behave incorrectly

[dackroyd]

The client variants of the logging interceptors were added in #33. Following this change, logging interceptors do not behave correctly in their default configurations i.e. without specifying Options as overrides

Logrus (server & client interceptors):

Unless WithLevels(...) is specified as an option, calls to the logging interceptor panic, as o.levelFunc is nil

Zap (client interceptor only):

Unless WithLevels(...) is specified as an option, calls to the logging interceptor are logged at the incorrect levels as the default is the DefaultCodeToLevel for both server and client interceptors

[mwitkow]

That was affecting master since #33 until #35. Now it's fixed, and the tests of #35 check the defaults.

Sorry about that.

[dackroyd]

Defaults look good again after #35 was merged, thank you!