Test cases needed for document_vector() and merge_wgts() in ulmfit

[bact]

These two functions need at least one test case:

pythainlp/pythainlp/ulmfit/core.py

Line 140 in ee67869

def document_vector(text: str, learn, data, agg: str = "mean"):

pythainlp/pythainlp/ulmfit/core.py

Line 198 in ee67869

def merge_wgts(em_sz, wgts, itos_pre, itos_new):

[wannaphong]

Now, document_vector test case is in #579

[wannaphong]

Done. Add test_merge_wgts 57f06e8