Bump to go 1.21

Change to reverseproxy.go to make tests pass came from @marsam on GitHub.

Dockerfile

@@ -1,6 +1,6 @@
 FROM golang:1.18.3-alpine
 WORKDIR /go/src/github.com/suyashkumar/ssl-proxy
 RUN apk add --no-cache make git zip
-RUN go get -u github.com/golang/dep/cmd/dep
 COPY . .
+RUN go get -u github.com/golang/dep/cmd/dep
 RUN make 

go.mod

@@ -1,6 +1,6 @@
 module github.com/suyashkumar/ssl-proxy
 
-go 1.18
+go 1.21
 
 require (
 	github.com/stretchr/testify v1.7.0

reverseproxy/reverseproxy.go

@@ -32,7 +32,7 @@ func newDirector(target *url.URL, extraDirector func(*http.Request)) func(*http.
 		} else {
 			req.URL.RawQuery = targetQuery + "&" + req.URL.RawQuery
 		}
-		if _, ok := req.Header["User-Agent"]; !ok {
+        if req.Header.Get("User-Agent") != "" {
 			// explicitly disable User-Agent so it's not set to default value
 			req.Header.Set("User-Agent", "")
 		}