mergeMaps should use the first params as default state

[albertwoo]

Giraffe.Swagger/src/Giraffe.Swagger/Common.fs

Line 46 in a525926

let mergeMaps (m1:Map<'k,'v>) (m2:Map<'k,'v>) =

This bug will cause httpHandler "produces" useless. Check the

Giraffe.Swagger/samples/SwaggerSampleApp/Program.fs

Line 181 in a525926

produces typeof<Car> ==>