Issue 3540: Fix baseUrl slash (#3546)

closes #3540 and #3545 

Tests:
```js
/// <reference types="Cypress" />

it('test url', function () {
    cy.visit('/')
    var baseUrl = Cypress.config('baseUrl')
    //baseUrl = baseUrl + '/'    // bug in Cypress - add slash
    cy.url().should('eq', Cypress.env('test_url'))
    cy.url().should('eq', baseUrl)
})

it('test url - copy paste from @jennifer-shehane', function () {
    cy.visit('/')
    cy.url().should('eq', Cypress.config('baseUrl'))
})
```

![obraz](https://user-images.githubusercontent.com/905878/53121397-10f83780-354c-11e9-8495-2474ced106e6.png)

packages/server/lib/config.coffee

@@ -230,8 +230,10 @@ module.exports = {
     .value()
 
     if url = config.baseUrl
-      ## always strip trailing slashes
-      config.baseUrl = _.trimEnd(url, "/")
+      ## replace multiple slashes at the end of string to single slash
+      ## so http://localhost/// will be http://localhost/
+      ## https://regexr.com/48rvt
+      config.baseUrl = url.replace(/\/\/+$/, "/")
 
     _.defaults(config, defaults)
 

packages/server/test/unit/config_spec.coffee

@@ -509,9 +509,34 @@ describe "lib/config", ->
     it "namespace=__cypress", ->
       @defaults "namespace", "__cypress"
 
+    it "baseUrl=http://localhost:8000/app/", ->
+      @defaults "baseUrl", "http://localhost:8000/app/", {
+        baseUrl: "http://localhost:8000/app///"
+      }
+
+    it "baseUrl=http://localhost:8000/app/", ->
+      @defaults "baseUrl", "http://localhost:8000/app/", {
+        baseUrl: "http://localhost:8000/app//"
+      }
+
     it "baseUrl=http://localhost:8000/app", ->
       @defaults "baseUrl", "http://localhost:8000/app", {
-        baseUrl: "http://localhost:8000/app//"
+        baseUrl: "http://localhost:8000/app"
+      }
+
+    it "baseUrl=http://localhost:8000/", ->
+      @defaults "baseUrl", "http://localhost:8000/", {
+        baseUrl: "http://localhost:8000//"
+      }
+
+    it "baseUrl=http://localhost:8000/", ->
+      @defaults "baseUrl", "http://localhost:8000/", {
+        baseUrl: "http://localhost:8000/"
+      }
+
+    it "baseUrl=http://localhost:8000", ->
+      @defaults "baseUrl", "http://localhost:8000", {
+        baseUrl: "http://localhost:8000"
       }
 
     it "javascripts=[]", ->