Normalize platformName in Appium helper and add unit test

lib/helper/Appium.js

@@ -261,11 +261,13 @@ class Appium extends Webdriver {
 
     this.platform = null
     if (config.capabilities[`${vendorPrefix.appium}:platformName`]) {
-      this.platform = config.capabilities[`${vendorPrefix.appium}:platformName`].toLowerCase()
+      config.capabilities[`${vendorPrefix.appium}:platformName`] = config.capabilities[`${vendorPrefix.appium}:platformName`].toLowerCase()
+      this.platform = config.capabilities[`${vendorPrefix.appium}:platformName`]
     }
 
     if (config.capabilities.platformName) {
-      this.platform = config.capabilities.platformName.toLowerCase()
+      config.capabilities.platformName = config.capabilities.platformName.toLowerCase()
+      this.platform = config.capabilities.platformName
     }
 
     return config

test/unit/Appium_platformNameNormalization_test.js

@@ -0,0 +1,55 @@
+const { expect } = require('chai')
+const Appium = require('../../lib/helper/Appium')
+
+describe('Appium platformName normalization', () => {
+  it('should normalize platformName to lowercase for Android', () => {
+    const app = new Appium({
+      platform: 'Android',
+      desiredCapabilities: {
+        platformName: 'Android',
+      },
+    })
+    const config = app._validateConfig({
+      platform: 'Android',
+      desiredCapabilities: {
+        platformName: 'Android',
+      },
+    })
+    expect(config.capabilities.platformName).to.equal('android')
+    expect(app.platform).to.equal('android')
+  })
+
+  it('should normalize platformName to lowercase for iOS', () => {
+    const app = new Appium({
+      platform: 'iOS',
+      desiredCapabilities: {
+        platformName: 'iOS',
+      },
+    })
+    const config = app._validateConfig({
+      platform: 'iOS',
+      desiredCapabilities: {
+        platformName: 'iOS',
+      },
+    })
+    expect(config.capabilities.platformName).to.equal('ios')
+    expect(app.platform).to.equal('ios')
+  })
+
+  it('should not change platformName if already lowercase', () => {
+    const app = new Appium({
+      platform: 'android',
+      desiredCapabilities: {
+        platformName: 'android',
+      },
+    })
+    const config = app._validateConfig({
+      platform: 'android',
+      desiredCapabilities: {
+        platformName: 'android',
+      },
+    })
+    expect(config.capabilities.platformName).to.equal('android')
+    expect(app.platform).to.equal('android')
+  })
+})