Lexer doesn't handle all white spaces

[evomassiny]

Hello,

By toying with this crate I stumble across a slight bug in the lexer,
using the current boa version, any non-" " white space character triggers a lexing error,
this code:

fn main() {
    // mind the '\t' in the js expression
    let mut lexer = boa::syntax::lexer::Lexer::new("\tconsole.log('foo');");
    let lex_res = lexer.lex().expect("Lexing failed");
}

fails and produce this stacktrace:

Finished dev [unoptimized + debuginfo] target(s) in 0.08s
     Running `target/debug/bug_demo`
thread 'main' panicked at '1:1: Unexpected '    '', /home/some_user/.cargo/registry/src/gh.neting.cc-1ecc6299db9ec823/Boa-0.4.0/src/lib/syntax/lexer.rs:576:23
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/gh.neting.cc-1ecc6299db9ec823/backtrace-0.3.34/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/gh.neting.cc-1ecc6299db9ec823/backtrace-0.3.34/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:47
   3: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:36
   4: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:200
   5: std::panicking::default_hook
             at src/libstd/panicking.rs:214
   6: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:477
   7: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:384
   8: std::panicking::begin_panic_fmt
             at src/libstd/panicking.rs:339
   9: boa::syntax::lexer::Lexer::lex
             at /home/some_user/.cargo/registry/src/gh.neting.cc-1ecc6299db9ec823/Boa-0.4.0/src/lib/syntax/lexer.rs:576
  10: pac_proxy::main
             at src/main.rs:34
  11: std::rt::lang_start::{{closure}}
             at /rustc/625451e376bb2e5283fc4741caa0a3e8a2ca4d54/src/libstd/rt.rs:64
  12: std::rt::lang_start_internal::{{closure}}
             at src/libstd/rt.rs:49
  13: std::panicking::try::do_call
             at src/libstd/panicking.rs:296
  14: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:80
  15: std::panicking::try
             at src/libstd/panicking.rs:275
  16: std::panic::catch_unwind
             at src/libstd/panic.rs:394
  17: std::rt::lang_start_internal
             at src/libstd/rt.rs:48
  18: std::rt::lang_start
             at /rustc/625451e376bb2e5283fc4741caa0a3e8a2ca4d54/src/libstd/rt.rs:64
  19: main
  20: __libc_start_main
  21: _start
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

According to the ECMASCRIPT specification, the \t should be simply ignored, as if it were a simple whitespace char.

Thank you for your time.