fix(@angular-devkit/build-angular): serve option merging is being overridden by defaults in schema

[alan-agius4]

We have a number of browser options that we allow the dev-server to merge. However, this only happens when such as options are undefined from the dev-server builder. At the moment these option have defaults inside their schema which results in them never being undefined, and hence the overridden logic is bypassed.

See:

angular-cli/packages/angular_devkit/build_angular/src/dev-server/index.ts

Lines 49 to 63 in 6dd5b18

	export const devServerBuildOverriddenKeys: (keyof DevServerBuilderOptions)[] = [
	'watch',
	'optimization',
	'aot',
	'sourceMap',
	'vendorSourceMap',
	'evalSourceMap',
	'vendorChunk',
	'commonChunk',
	'baseHref',
	'progress',
	'poll',
	'verbose',
	'deployUrl',
	];

and

angular-cli/packages/angular_devkit/build_angular/src/dev-server/index.ts

Lines 107 to 115 in 6dd5b18

	const overrides = (Object.keys(options) as (keyof DevServerBuilderOptions)[])
	.filter(key => options[key] !== undefined && devServerBuildOverriddenKeys.includes(key))
	.reduce<json.JsonObject & Partial<BrowserBuilderSchema>>(
	(previous, key) => ({
	...previous,
	[key]: options[key],
	}),
	{},
	);

Fixes #15273 and fixes #15064

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