fix: writeFile returns string instead JSON (#2373)

* Returning obj instead of text

If input of writeFile is an object, it should return an object after successful write instead of a string.

* Testing returned JSON of writeFile

packages/driver/src/cy/commands/files.coffee

@@ -94,13 +94,15 @@ module.exports = (Commands, Cypress, cy, state, config) ->
         })
 
       if _.isObject(contents)
+        objContents = contents
         contents = JSON.stringify(contents, null, 2)
 
       Cypress.backend("write:file", fileName, contents, _.pick(options, ["encoding", "flag"]))
       .then ({ contents, filePath }) ->
         consoleProps["File Path"] = filePath
         consoleProps["Contents"] = contents
-
+        if objContents?
+          return objContents
         return contents
       .catch Promise.TimeoutError, (err) ->
         $utils.throwErrByPath "files.timed_out", {

packages/driver/test/cypress/integration/commands/files_spec.coffee

@@ -291,6 +291,12 @@ describe "src/cy/commands/files", ->
 
       cy.writeFile("foo.txt", "contents").then (subject) ->
         expect(subject).to.equal("contents")
+
+    it "sets a JSON as the subject", ->
+      Cypress.backend.resolves(okResponse)
+
+      cy.writeFile("foo.json", { name: "Test" }).then (subject) ->
+        expect(subject.name).to.equal("Test")
 
     it "can write a string", ->
       Cypress.backend.resolves(okResponse)