fix: Build the URL using url.JoinPath instead of path.Join (#249)

## Which problem is this PR solving?

There is a weird bug in setting a URL path directly, where it will
escape some characters, but not all.

This [go playground](https://go.dev/play/p/QnMDXnU9lDq) example shows
it. The unescaped value is used directly, the escaped value double
escapes some of the characters.

This PR uses url.PathJoin instead, which is intended to handle URLs.

## Short description of the changes
- Update the URL builder to use url.JoinPath and return any errors
- Update Transmission to use updated builder func and handle error like
before
- Update tests to handle errors

transmission/transmission.go

@@ -19,7 +19,6 @@ import (
 	"io/ioutil"
 	"net/http"
 	"net/url"
-	"path"
 	"runtime"
 	"strings"
 	"sync"
@@ -399,7 +398,7 @@ func (b *batchAgg) fireBatch(events []*Event) {
 		}
 	}
 
-	url, err := url.Parse(apiHost)
+	url, err := buildRequestURL(apiHost, dataset)
 	if err != nil {
 		end := time.Now().UTC()
 		if b.testNower != nil {
@@ -421,9 +420,6 @@ func (b *batchAgg) fireBatch(events []*Event) {
 		return
 	}
 
-	// build the HTTP request
-	url.Path = buildReqestPath(url.Path, dataset)
-
 	// One retry allowed for connection timeouts.
 	var resp *http.Response
 	for try := 0; try < 2; try++ {
@@ -437,9 +433,9 @@ func (b *batchAgg) fireBatch(events []*Event) {
 			// Pass the underlying bytes.Reader to http.Request so that
 			// GetBody and ContentLength fields are populated on Request.
 			// See https://cs.opensource.google/go/go/+/refs/tags/go1.17.5:src/net/http/request.go;l=898
-			req, err = http.NewRequest("POST", url.String(), &reader.Reader)
+			req, err = http.NewRequest("POST", url, &reader.Reader)
 		} else {
-			req, err = http.NewRequest("POST", url.String(), reqBody)
+			req, err = http.NewRequest("POST", url, reqBody)
 		}
 		req.Header.Set("Content-Type", contentType)
 		if zipped {
@@ -743,6 +739,6 @@ type nower interface {
 	Now() time.Time
 }
 
-func buildReqestPath(existingPath, dataset string) string {
-	return path.Join(existingPath, "/1/batch", url.PathEscape(dataset))
+func buildRequestURL(apiHost, dataset string) (string, error) {
+	return url.JoinPath(apiHost, "/1/batch", url.PathEscape(dataset))
 }

transmission/transmission_test.go

@@ -1395,7 +1395,8 @@ func TestBuildRequestPath(t *testing.T) {
 	}
 
 	for _, tc := range testCases {
-		path := buildReqestPath("", tc.datasetName)
-		assert.Equal(t, tc.expectedPath, path)
+		url, err := buildRequestURL("http://fakeHost:8080", tc.datasetName)
+		assert.NoError(t, err)
+		assert.Equal(t, "http://fakeHost:8080"+tc.expectedPath, url)
 	}
 }