Fix throwing exception when Trailers is not available

[azyobuzin]

Rin.Middlewares.RequestRecorderMiddleware sometimes outputs error logs saying:

Unhandled Exception was thrown until post-processing

System.InvalidOperationException
The request trailers are not available yet. They may not be available until the full request body is read.
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.Microsoft.AspNetCore.Http.Features.IHttpRequestTrailersFeature.get_Trailers()
   at Rin.Middlewares.RequestRecorderMiddleware.PostprocessAsync(HttpContext context, RinOptions options, HttpRequestRecord record)
   at Rin.Middlewares.RequestRecorderMiddleware.InvokeAsync(HttpContext context, RinOptions options)

This exception is thrown when IHttpRequestTrailersFeature feature is available but IHttpRequestTrailersFeature.Available is false. So checking whether that property is true is needed to read trailer.

This pull request has these changes:

• Read Trailers property after reading the body to let the HTTP server read trailer
• Use CheckTrailersAvailable instead of SupportsTrailers to prevent failure to read Trailers

[mayuki]

Thanks! Looks yosasou to me.