Use http.StatusContinue constant instead of magic number 100

Replace magic number `100` with `http.StatusContinue` constant for better code clarity in `bodyAllowedForStatus` function.

Fixes #4489

.codecov.yaml

@@ -0,0 +1,7 @@
+codecov:
+  coverage:
+    status:
+      project:
+        enabled: false
+      patch:
+        target: 99%

context.go

@@ -1058,7 +1058,7 @@ func (c *Context) requestHeader(key string) string {
 // bodyAllowedForStatus is a copy of http.bodyAllowedForStatus non-exported function.
 func bodyAllowedForStatus(status int) bool {
 	switch {
-	case status >= 100 && status <= 199:
+	case status >= http.StatusContinue && status <= 199:
 		return false
 	case status == http.StatusNoContent:
 		return false