tweak: type checking and warn methods that conflict with internals

[toBeTheLight]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe: better checking and warn info

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[yyx990803]

Please provide more information on why these changes are needed.

[toBeTheLight]

1. The first change is adding warn info when key of data is reserved. When key is reserved, code ignore the key without giving a warn.
2. The second change is about type checking of method value. Code methods[key] != null was added in pull request#3656 to checking for undefined properties.
   But when we call the method which type is number or string, not function, it will give a ambiguous error message too.
   And in the api document, it says type of method is function, so I changed methods[key] != null to typeof methods[key] !== 'function', and changed the warn info from has an undefined value to is not a function