Improve wasm-themis to be more compatible with TypeScript

[radetsky]

In secure_cell_seal mode two encrypt method and one decrypt was declared with a single parameter and the developer used optional arguments to handle the optional context. When we use TypeScript to work with the Themis, the compiler generates the error while compiling in case of using a non-empty context.

I made a declaration of an optional parameter, not an argument (see javascript parameters vs arguments of function). Also, I remove a single case of test cases (see test.js diff) because undefined optional context is always ok. We can use it.

Checklist

• Change is covered by automated tests
• The coding guidelines are followed
• Public API has proper documentation
• Example projects and code samples are up-to-date (in case of API changes)
• Changelog is updated (in case of notable or breaking changes)