fix(subscribable): make subscribe() signature match Observable

[felixfbecker]

Description:
The signature of Subscribable.subscribe() did not match that of Observable.subscribe(), which made the two types not compatible if T=never (see linked issue).

It also was wrong, because it allowed passing both an Observer and functions at the same time.

This makes it match the signature of Observable.subscribe() exactly, which solves the type error in the issue.

It is not breaking (unless you passed both observer and functions, which is extremely unlikely and did not work at runtime anyway as SafeSubscriber checks if the first argument is a function and if yes, discards the other arguments, so this would only highlight a bug).

The alternative discussed in the issue was removing the 3-arg overload, but the discussion in the spec does not seem resolved yet, it is not clear that the interface is supposed to represent the spec, it would be a breaking change and require way more changes in rxjs (it did not compile when I tried).

Related issue (if exists):
Fixes #3891

cc @cartant

[ci-reporter]

The build is failing

✨ Good work on this PR so far! ✨ Unfortunately, the Travis CI build is failing as of 2ecf10b. Here's the output:

npm test

> @reactivex/rxjs@6.2.2 test /home/travis/build/ReactiveX/rxjs
> cross-env TS_NODE_PROJECT=spec/tsconfig.json mocha --opts spec/support/default.opts "spec/**/*-spec.ts"


/home/travis/build/ReactiveX/rxjs/node_modules/ts-node/src/index.ts:250
    return new TSError(diagnosticText, diagnosticCodes)
           ^
TSError: ⨯ Unable to compile TypeScript:
spec/operators/zipAll-spec.ts(42,28): error TS2365: Operator '+' cannot be applied to types 'string | number' and 'string | number'.
spec/operators/zipAll-spec.ts(240,38): error TS2345: Argument of type 'OperatorFunction<ObservableInput<number>, number>' is not assignable to parameter of type 'OperatorFunction<number[] | HotObservable<string>, number>'.
  Type 'number[] | HotObservable<string>' is not assignable to type 'ObservableInput<number>'.
    Type 'HotObservable<string>' is not assignable to type 'ObservableInput<number>'.
      Type 'HotObservable<string>' is not assignable to type 'Iterable<number>'.
        Property '[Symbol.iterator]' is missing in type 'HotObservable<string>'.
spec/operators/zipAll-spec.ts(381,28): error TS2365: Operator '+' cannot be applied to types 'string | number' and 'string | number'.

    at createTSError (/home/travis/build/ReactiveX/rxjs/node_modules/ts-node/src/index.ts:250:12)
    at getOutput (/home/travis/build/ReactiveX/rxjs/node_modules/ts-node/src/index.ts:358:40)
    at Object.compile (/home/travis/build/ReactiveX/rxjs/node_modules/ts-node/src/index.ts:545:11)
    at Module.m._compile (/home/travis/build/ReactiveX/rxjs/node_modules/ts-node/src/index.ts:430:43)
    at Module._extensions..js (module.js:663:10)
    at Object.require.extensions.(anonymous function) [as .ts] (/home/travis/build/ReactiveX/rxjs/node_modules/ts-node/src/index.ts:433:12)
    at Module.load (module.js:565:32)
    at tryModuleLoad (module.js:505:12)
    at Function.Module._load (module.js:497:3)
    at Module.require (module.js:596:17)
    at require (internal/module.js:11:18)
    at /home/travis/build/ReactiveX/rxjs/node_modules/mocha/lib/mocha.js:231:27
    at Array.forEach (<anonymous>)
    at Mocha.loadFiles (/home/travis/build/ReactiveX/rxjs/node_modules/mocha/lib/mocha.js:228:14)
    at Mocha.run (/home/travis/build/ReactiveX/rxjs/node_modules/mocha/lib/mocha.js:536:10)
    at Object.<anonymous> (/home/travis/build/ReactiveX/rxjs/node_modules/mocha/bin/_mocha:582:18)
    at Module._compile (module.js:652:30)
    at Object.Module._extensions..js (module.js:663:10)
    at Module.load (module.js:565:32)
    at tryModuleLoad (module.js:505:12)
    at Function.Module._load (module.js:497:3)
    at Function.Module.runMain (module.js:693:10)
    at startup (bootstrap_node.js:191:16)
    at bootstrap_node.js:612:3

I'm sure you can fix it! If you need help, don't hesitate to ask a maintainer of the project!

---

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[felixfbecker]

I wish rxjs could just get rid of all the unneeded project functions

[coveralls]

Coverage decreased (-0.2%) to 96.783% when pulling 2706b1a on felixfbecker:fix-subscribable into 9ea0ec9 on ReactiveX:master.