Fix/handle initial digit of operation (oapi-codegen#735)

* test: internal/test/issues/issue-head-digit-of-operation-id

* fix: escape digit from head of operationId

* chore: add issue.gen.go(head-digit-of-operation-id)

* fix: use typeNamePrefix() to remove head digit

* stop direct writing for less confusion.
* cd ./internal/test; go generate ./...

* chore: make generate with go 1.16.9

internal/test/issues/issue-head-digit-of-operation-id/config.yaml

@@ -0,0 +1,7 @@
+---
+package: head_digit_of_operation_id
+generate:
+  strict-server: true
+output: issue.gen.go
+output-options:
+  skip-prune: true

internal/test/issues/issue-head-digit-of-operation-id/doc.go

@@ -0,0 +1,3 @@
+package head_digit_of_operation_id
+
+//go:generate go run github.com/deepmap/oapi-codegen/cmd/oapi-codegen --config=config.yaml spec.yaml

internal/test/issues/issue-head-digit-of-operation-id/spec.yaml

@@ -0,0 +1,7 @@
+openapi: 3.0.2
+info:
+  version: "0.0.1"
+paths:
+  /3gpp/foo:
+    get:
+      operationId: 3GPPFoo

pkg/codegen/operations.go

@@ -510,10 +510,10 @@ func OperationDefinitions(swagger *openapi3.T) ([]OperationDefinition, error) {
 					return nil, fmt.Errorf("error generating default OperationID for %s/%s: %s",
 						opName, requestPath, err)
 				}
-				op.OperationID = op.OperationID
 			} else {
 				op.OperationID = ToCamelCase(op.OperationID)
 			}
+			op.OperationID = typeNamePrefix(op.OperationID) + op.OperationID
 
 			// These are parameters defined for the specific path method that
 			// we're iterating over.