Update dependency helmet to v3.19.0

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
helmet (source)	dependencies	minor	3.16.0 -> 3.19.0

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Release Notes

helmetjs/helmet

### [`v3.19.0`](https://togithub.com/helmetjs/helmet/blob/master/CHANGELOG.md#​3190---2019-07-17) [Compare Source](https://togithub.com/helmetjs/helmet/compare/v3.18.0...v3.19.0) ##### Updated - Updated `dns-prefetch-control` to v0.2.0 - Updated `dont-sniff-mimetype` to v1.1.0 - Updated `helmet-crossdomain` to v0.4.0 - Updated `hide-powered-by` to v1.1.0 - Updated `x-xss-protection` to v1.2.0 ### [`v3.18.0`](https://togithub.com/helmetjs/helmet/blob/master/CHANGELOG.md#​3180---2019-05-05) [Compare Source](https://togithub.com/helmetjs/helmet/compare/v3.17.0...v3.18.0) ##### Added - `featurePolicy` has 19 new features: `ambientLightSensor`, `documentDomain`, `documentWrite`, `encryptedMedia`, `fontDisplayLateSwap`, `layoutAnimations`, `legacyImageFormats`, `loadingFrameDefaultEager`, `oversizedImages`, `pictureInPicture`, `serial`, `syncScript`, `unoptimizedImages`, `unoptimizedLosslessImages`, `unoptimizedLossyImages`, `unsizedMedia`, `verticalScroll`, `wakeLock`, and `xr` ##### Changed - Updated `expect-ct` to v0.2.0 - Updated `feature-policy` to v0.3.0 - Updated `frameguard` to v3.1.0 - Updated `nocache` to v2.1.0 ### [`v3.17.0`](https://togithub.com/helmetjs/helmet/blob/master/CHANGELOG.md#​3170---2019-05-03) [Compare Source](https://togithub.com/helmetjs/helmet/compare/v3.16.0...v3.17.0) ##### Added - `referrerPolicy` now supports multiple values ##### Changed - Updated `referrerPolicy` to v1.2.0

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