consider if has_consistent_fields_per_line() always reports the truth

[cjyetman]

In RMI-PACTA/pacta.portfolio.analysis#175, guess_delimiter() was made to use stringr::str_count() to count the number of delimiters per line rather than use strsplit() to count the number of fields because strsplit() drops the last field if it is blank (or was removed because it's quoted). has_consistent_fields_per_line() may benefit from a similar treatment

if/when RMI-PACTA/pacta.portfolio.analysis#174 is complete, this issue should be transferred to pacta.portfolio.import

AB#10855