Support IEnumerable<KeyValuePair<string, string>> in SerilogLoggerScope

[admalledd]

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In EnrichAndCreateScopeItem() if my TState was originally created as Dictionary<string, string> the if statement fails to pivot my items into LogEvent.Properties as expected.

Similar to #186 but not quite, though if work on either is done, probably best to do both enhancements?

A reason why I am using <string, string> is that I want to be sure at time of BeginScope that all key/values are properly string-ified as expected for eventual SQL (or other tablized, like Splunk, AzureTableStorage...) column-store.