Fix keeping old attribs in bs_set_attr()

[pvictor]

Hello,

Little change to keep previous attributes in a tag in bs_set_attr(), especially when the attribute is multiple :

tag <- tags$div(class = "class1", class = "class2")
tag
# <div class="class1 class2"></div>
tag$attribs
# $class
# [1] "class1"
# 
# $class
# [1] "class2"

Old & new behavior :

tag <- htmltools::tags$div(class = "class1", class = "class2")
## old : class2 is dropped
bsplus:::bs_set_attr(tag, target = "#target")
# <div class="class1" data-target="#target"></div>
## new : class 2 is preserved
bsplus:::bs_set_attr(tag, target = "#target")
# <div class="class1 class2" data-target="#target"></div>

It fixes one issue in shinyWidget : https://github.com/dreamRs/shinyWidgets/issues/215 And will solve this one in bsplus : https://github.com/ijlyttle/bsplus/issues/55

Victor

[ijlyttle]

This looks great, thanks!